The present invention relates to the arrangement wherein a power transmission element includes a preformed interface element that is joined to a carrier element via deformation of the carrier element. The preformed interface element includes a surface that contacts a mating external power transmission element. More specifically, this invention focuses on a preformed driving sprocket and a sprocket carrier element, wherein the sprocket carrier element, or portion thereof, is deformed to encapsulate or otherwise connect to a portion of the preformed driving sprocket. This invention is particularly adaptable to the sprocket cluster of a bicycle wheel where a multiplicity of sprockets may be captured within a common carrier or sprocket hub.
In general, it should be recognized that a mechanical drivetrain consists of a plurality of mechanical power transmission elements, including at least one initial input element and at least one final output element and often at least one intermediate element situated between the input and output elements. From the perspective of any individual power transmission element, power, or the motion associated with power, is input to this individual element via interaction at a mating interface with an external input element and motion is output to a mating external output element via a second mating interface. In some cases, motion may be output to a plurality of external output elements, including one external power transmission element and one auxiliary power transmission element. Often, the final output element of the drivetrain does not transmit motion to an external element since, by definition, the desired motion has been achieved.
For the past century, motive force for bicycles has been transmitted through the rear wheel via a drivetrain that includes a roller chain and sprocket transmission system. With the advent of the derailleur transmission, a variable driving ratio was created by selectively engaging the roller chain with any one of several axially spaced sprockets, all connected to the same hub. Current bicycle technology utilizes multiple sprockets on the driving axle, which is normally connected to the pedal crankshaft, as well as on the driven axle, which is normally connected to the rear wheel. Generally, the driven axle of the rear wheel includes a sprocket assembly with as many as nine sprockets, all connected to the same wheel hub. This wheel hub usually includes a freewheeling clutch to transmit driving torque only in the forward direction of rotation while slipping or freewheeling in the reverse direction.
The technology for the rear wheel sprocket assembly, has evolved over the years. Initially, the sprockets were fixed together to include the freewheeling clutch in one unit, called a xe2x80x9cfreewheelxe2x80x9d, which was then assembled to the hub of the rear wheel. While the xe2x80x9cfreewheelxe2x80x9d is still in use today, the state-of-the-art drive mechanism includes an assembly whereby the clutch is incorporated into the rear hub of the bicycle wheel prior to attachment of the sprockets. Such an assembly is termed a xe2x80x9cfreehubxe2x80x9d. The clutch portion of the xe2x80x9cfreehubxe2x80x9d is termed the xe2x80x9cfreehub bodyxe2x80x9d and includes a splined outer shell over which the sprockets or sprocket assembly is assembled. The sprockets include splines on their inside diameter to mate with the splines of the freehub body, thereby transmitting driving torque from the sprocket to the wheel hub. The sprocket assembly, or xe2x80x9ccassettexe2x80x9d, is then assembled to the freehub body, either as a series of individual sprockets and spacers or as a subassembly where the sprockets are first affixed to a carrier, which is then assembled over the splines of the freehub body. A lockring is used to secure the cassette to the freehub body.
Whether a freewheel or a freehub arrangement is used, the assembly is generally quite heavy due to the weight of the metal components that are usually fabricated from steel. In addition, the parts of the sprocket assembly are generally quite expansive since they are constructed from a relatively large number of individual metal components, each of which must be separately machined or cast and finally assembled together.
The present invention involves a power transmission element, including a separately formed sprocket portion that is affixed to a central carrier portion via the deformation of the carrier material. In a preferred embodiment of the present invention, the separately formed sprocket is affixed to a central carrier via solidification of fluent carrier material to engage or capture the sprocket. In manufacture of this preferred embodiment, at least one preformed sprocket is fixtured within a mold cavity such that a portion of the sprocket protrudes within the mold cavity. Subsequently, molten thermoplastic polymer is injection molded to fill the mold cavity and surround the protruding portion of the sprocket. Once the polymer cools and solidifies, the combined polymer/sprocket assembly is ejected from the mold, yielding a composite sprocket assembly with exposed metal sprocket teeth and a polymer central hub or carrier portion. During this molding process, the fluent carrier material may be considered to deform to encapsulate and capture the preformed sprocket.
Thus, the toothed sprocket perimeter, the portion of the sprocket that engages the drive chain, is left exposed and may be of sufficiently hard material to withstand the wear and contact stresses associated with the transmission of motive force between these two power transmission elements. Conversely, the central carrier or hub portion of the sprocket, which does not experience the same wear or contact stress, may be made from a softer or weaker material, such as polymeric material, that is preferably also lighter in weight and lower in cost. Fiber reinforced polymers and particle reinforced polymers are materials particularly suited in this application, since they are inexpensive, relatively strong, light in weight, easy to process and have good dimension control.
It is preferable that the carrier portion of the sprocket be wider than the sprocket thickness, as measured axially. This allows the softer teeth on the inside diameter of the carrier to contact the freehub body splines over a wider region, serving to distribute the contact forces between the sprocket hub and the freehub body over a wider area. As this area of contact is increased, the contact stress between the two components is reduced.
Further, the increased width of the sprocket hub may be sized such that, when the adjacent sprocket is assembled to the freehub body, the sprockets are stacked such that proper axial spacing is maintained between the adjacent sprockets. This eliminates the need for additional spacers between the sprockets.
A preferred embodiment focuses on encapsulation of one or more sprockets within a polymer hub such that the polymer material engages the sprocket(s) and transmits the torque to the axle or other components of the wheel hub. The greatest benefit is obtained when a plurality of axially spaced sprockets are encapsulated within a single carrier. Since the chain is engaged with only one sprocket at a given time, the driving torque may be transmitted across the full width of the carrier, distributing the load and reducing the stress within the polymer carrier material.
In addition, the carrier may now serve to retain a multiple of sprockets together, thus eliminating the need for individual spacers between the sprockets and reducing the number of separate parts that must be assembled or disassembled for servicing. In addition to the added convenience associated with reducing the number of parts, the cost is also reduced as parts are eliminated, as compared to the prior art assemblies. Since the carrier is now created in a single net-shape molding process, the multiplicity of forming and assembly operations associated with the individual components of the prior art assembly is reduced, reducing cost still further. Since the stresses due to torque transmission are now distributed over a larger portion of carrier material, stresses are reduced and lower strength carrier materials may be utilized. Such materials are usually also lower in cost.
Weight reduction is often a primary concern for performance-oriented cyclists. Since the present invention may utilize a carrier of lightweight polymer material and since the central opening of the preformed sprockets is larger, the overall volume of steel in the sprockets may be reduced, thereby providing a significant weight saving benefit.
Often the axial spacing distance between adjacent sprockets must be accurately controlled to insure optimal derailleur shifting performance from one sprocket to the next. Prior art cassette assemblies include a stackup of thickness tolerances among the multitude of individual sprocket and spacer components, making closely controlled spacing difficult and expensive to achieve. The present invention, on the other hand, uses a precision mold to precisely and repeatably locate the sprockets, resulting in greater spacing accuracy at a reduced cost.
The steel sprockets are most often blanked out of sheet metal in a profile stamping process. Since the present invention permits these sprockets to have a much larger central opening, the central blanked slug, which is normally wasted, is now large enough to be utilized to create a smaller sprocket. Thus, the same footprint of sheetmetal may yield two or more sprockets, reducing the raw material cost.
In the case where the polymer carrier material is prone to yielding or deformation at the interface with the freehub body splines, additional inserts may be added to distribute the contact stresses over a broader surface area of the polymer carrier. Such an insert may be encapsulated and retained within the carrier or it may be assembled between the freehub body and the sprocket carrier subsequent to molding.
Since the carrier is molded in a net-shape forming process, a wide variety of additional features may be incorporated within the carrier itself with little or no additional cost. Some of these features include shifting ramps, screw bosses and auxiliary components.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.